The use of an electric shield, also referred to as an electrostatic shield, between primary and secondary coils or windings is well known in the art, such as in power supply and telecommunications applications. An electric shield can be used to prevent a high voltage breakdown from the primary winding to the secondary winding. For example, in the case of the presence of lightning impulse voltage the electric shield conducts the breakdown to ground. An electric shield can also be used to attenuate electrical noise that would otherwise be coupled between the primary and secondary windings.
FIG. 1 shows an elevational view (1A) and an exploded view (1B) of a conventional iron core transformer that has an electric shield, as shown and described in US 2003/0030534 A1, “Transformer Shielding”, Gu et al. FIG. 2 shows the electric shield of FIG. 1 in greater detail. Note that the shield is not provided as a continuous sheet of metal, but instead contains a number of cut-out areas forming a comb-type pattern of parallel, electrically conductive traces connected together on one end by a connecting trace. The purpose of using the comb-type shape is to prevent the formation of eddy currents. The presence of eddy currents is disadvantageous, as they increase the losses that occur between the primary and the secondary windings.
FIG. 3 depicts an equivalent circuit of a conventional noise-shielded transformer, such as one shown in U.S. Pat. No. 5,150,046. The shield is shown as being connected to circuit ground. During operation the high frequency magnetic flux in the primary winding, generated by pulse noise, is minimized by being directed to ground through the static capacitance C of the shield winding
It is known to use an electric shield under a planar integrated inductor. The electric shield in this case is typically referred to in the literature as a ground shield. The purpose of the ground shield is to prevent noise coupling from a conductive substrate (e.g., bulk silicon). Another purpose of the ground shield is to increase the quality (Q) factor of the inductor, as the lossy ground capacitance is reduced by the presence of the electric shield.
FIGS. 4A–4G, collectively referred to as FIG. 4, show plan view schematics of different conventional metal ground shield structures (FIGS. 4C–4F), in comparison to a floating and a grounded silicon substrate shown in FIGS. 4A and 4B, respectively, and to a solid metal ground plate shown in FIG. 4G. Reference can also be made to: “Progress in RF inductors on silicon-understanding substrate losses”, Burghartz, IEEE IEDM'98, pp. 523–526.
FIG. 5 shows an example of a ground shield that can be placed under a planar inductor or a transformer. Reference in this regard can be made to FIG. 7 of commonly assigned US 2003/0071706 A1, “Planar Transformers”, by Christensen. In this case an integrated transformer is disposed over the patterned ground shield. The ground shield comprises an array of generally radially extending fingers connected by a broken ring. The broken ring is positioned some distance inwardly from the outer periphery of the ground shield, in a region where the transformer's magnetic field is parallel to the surface of the substrate on which the transformer is formed. Positioning the connecting ring in this way is said to reduce the series resistance of the shield, when compared with a similar shield with a peripherally-disposed connecting ring.
Also of interest is US 2003/0001713, “Integrated Transformer”, by Gardner. In this US Patent a structure is disclosed to include magnetic layers, and thus would have a ferrite core which acts to strengthen the magnetic coupling. The magnetic layer is slotted radially, i.e. perpendicular to the windings, and the slots are used to reduce eddy currents. In this approach, however, the magnetic layers are not disclosed to be used as an electric shield, but simply as a magnetic core.
In US2001/0050607 A1, “Integrated Transformer”, Gardner, a structure is baked from dielectric, magnetic and conducting layers. The magnetic layer is composed of an amorphous cobalt alloy, and as such the structure has a ferrite core. An electric shield is not disclosed as forming part of the structure.
In U.S. Pat. No. 6,580,334 B2, “Monolithically Integrated Transformer”, Simbürger et al. disclose a transformer that is said to be produced according to standard silicon bipolar technology with three metallic layers. The transformer is not disclosed to contain an electric shield.
In U.S. Pat. No. 5,877,667, “On-Chip Transformers”, Wollensen discloses embodiments of transformers constructed in separate metal layers in an insulator that serves as a dielectric. The insulator layer is formed on a silicon substrate. The use of an electric shield is not disclosed.
In U.S. Pat. No. 6,031,445, “Transformer for Integrated Circuits”, Marty et al. disclose a transformer constructed from four layers of conductive lines separated by insulating layers. The transformer structure does not include an electric shield.
Of most interest to this invention are integrated planar transformers. A planar, integrated transformer 1, or a balun, is basically two planar coils 2 and 3 inter-wound on the same layer (FIG. 6) or on different layers (FIG. 7). The transformer 1 may be considered to be two-mutually-coupled inductors (L1, L2), as shown in FIG. 8. The inductors L1, L2 are coupled to each other with a coupling factor K. An ideal 1:1 transformer 1 has a coupling factor of K=1, while when the coupling is not perfect the coupling factor is less than unity. Mutual inductance is another way to represent the inductive coupling, and corresponds to the coupling factor in the form M=K(sqrroot(L1L2)).
Considering the effect of K less than unity, the circuit can be shown as in FIG. 9 in an equivalent presentation, in which a pair of mutual inductances with an inductance equal to the original M are coupled perfectly to each other, i.e., K=1. In addition, serial inductances Ls1 and Ls2, having values L-M, are considered to be uncoupled from one another, and are added in series with the inductances L1′ and L2′ of the ideal transformer.
In addition to the non-perfect inductive coupling the integrated transformer has capacitance between the primary to the secondary coils.
From the circuit design point of view it would be advantageous to construct a transformer with a high K and negligible capacitance between the primary and the secondary coils. In practice, however, the effect of the low K changes the function of the circuit from a transformer to one resembling a serial-shunt-serial inductance circuit. While such a circuit can be matched (with some difficulty), it typically exhibits a narrowband frequency response. The inherent capacitance can also be significant and may cause, for example, common mode leakage through the transformer. When a transformer is used as a balun the capacitance between the primary and secondary windings can result in an asymmetry in the impedance as seen from the balanced port.
Further, in order to improve a planar integrated transformer in accordance with the prior art could require that magnetic core material be somehow added. As can be appreciated, this would result in additional and possibly non-standard process steps, and would result in increased cost and complexity, and possibly in reduced yield.
Based on the foregoing representative sampling of the art and related discussion, it can be appreciated that a need exists for a transformer that can be constructed using electrically conductive elements disposed in layers of an integrated circuit, and that further includes an electric shield disposed between windings to provide the benefits that accrue from the use of the electric shield. Prior to this invention, this need was not satisfied.